The present invention relates to a hydro-mechanical transmission having both mechanical and hydrostatic power branches and in particular to such a transmission for use in an agricultural tractor.
The general problem in transferring power from engines to the ground, whether in agricultural tractors or road vehicles, is that different ratios are needed between the engine and wheels or tracks to match various operating conditions. Traditionally this has been done with discrete gear ratios. These ratios can be selected manually, as in conventional manual transmissions, or with operator controlled powershift transmissions. The ratios can also be controlled by the transmission as in automatic transmissions commonly found in automobiles. However, there are a limited number of discrete ratios available and the available ratios are not always optimum.
Accordingly, stepless or infinitely variable transmissions are desirable. Such transmissions are commonly embodied in hydrostatic drives, such as those found in self-propelled agricultural combines and lawn tractors. Another solution is electric drive as is used in railway locomotives and in some earth moving equipment. Both solutions have cost and efficiency disadvantages.
Another solution is the hydro-mechanical transmission. Hydro-mechanical transmissions are transmissions that combine a mechanical transmission with a hydrostatic unit. Although mechanical transmissions are generally more efficient and reliable than pure hydrostatic transmissions, they have the disadvantage of not being infinitely variable, as are the more expensive hydrostatic transmissions. Likewise, the hydrostatic transmissions have a major disadvantage in that they are less efficient than mechanical transmissions. Hydrostatic transmissions also generally require larger components, such as larger pumps and motors, as the transmission increases in capacity. The general advantage of hydro-mechanical transmissions over hydrostatic drives is that part of the power is transmitted mechanically, resulting in better efficiency than pure hydrostatic drives.
In order to satisfy space limitations, reduce cost, increase efficiency and provide an infinitely variable speed, hydro-mechanical transmissions have been developed that combine the best features of both transmissions. Hydro-mechanical transmissions are typically of a split power input type, where a hydrostatic unit and a mechanical transmission are driven in parallel by the vehicle engine. The hydrostatic output power is combined in the mechanical transmission with the split mechanical power input from the engine to produce hydro-mechanical output power in multiple power ranges. In each range, varying the stroke of the hydrostatic unit can infinitely vary the speed and torque.
However, previous attempts at using hydro-mechanical transmissions in agricultural tractors have had some disadvantages. Some hydrostatic transmissions require that the vehicle be at a standstill to shift between low and high range. Others have a high degree of complexity, while still others have the mode shift point in the field working range and the transmission efficiency at the shift point will be lower than the efficiency between mode shifts. Many hydro-mechanical transmissions also require additional gear sets to provide a reverse range.
In the known art there are basically three types of hydro-mechanical cycles. The first is the input coupled or torque split type. This type has one hydrostatic unit coupled or geared to the input of the transmission. The hydrostatic unit is generally a variable displacement unit. The other hydrostatic unit is coupled or geared to the torque splitting planetary. This unit is often a fixed displacement unit. The second is the output coupled or speed split. In this type of arrangement one hydrostatic unit is coupled to the output of the transmission and the other unit is coupled or geared to the torque splitting planetary. In practice both units are generally variable displacement units. The third is the four shaft or compound type. In this type neither hydrostatic unit is coupled to the transmission input or output. Instead both units are coupled or geared to the torque splitting planetary. There may be one or more than one planetary gear set involved.
Most hydro-mechanical transmissions use only one type of hydro-mechanical cycle. Of these, the most commonly used type is the input coupled or torque split type. These transmissions differ in the number of modes and in the arrangement of gears, but the hydro-mechanical cycle is the same. It has been known to utilize all three cycles in a single transmission. Known transmissions of this type have three modes of which the first is output coupled, the second is compound and the third is the non-regenerative part of the input coupled cycle. The advantage of this type of transmission is that both mode shifts are synchronous, zero torque shifts. The major disadvantage of such transmissions is that for a given power level the hydrostatic units are larger than in multiple mode input coupled transmissions.
Accordingly, there is a clear need in the art for a hydro-mechanical transmission that is compact and is capable of infinitely variable operation within its speed range without a significant drop in efficiency in the working ranges.
It is an object of the present invention to provide a hydro-mechanical transmission that is uniquely designed for optimal operation in an agricultural tractor.
It is a further advantage of the present invention to provide a transmission having a physical package similar to existing mechanical transmissions in length, enabling it to be readily integrated into existing tractor designs with minimal modification to the tractor. Many prior hydro-mechanical transmissions do not integrate a reverse gear set into the combining mechanical transmission but instead have a separate gear set before or after the combining mechanical transmission for shifting between forward and reverse directions. Such a design takes more space and may require the vehicle be brought to a stop before shifting between forward and reverse.
The four mode hydro-mechanical transmission disclosed herein has a planetary system including three planetary gear sets together with four clutches and a reverse brake to provide an infinitely adjustable speed change without an additional direction change gear set. A first hydrostatic unit is geared to the second row ring of the torque splitting planetary. A second hydrostatic unit is geared to the planet carrier via a first clutch and a gear pair in Modes One and Four. The second hydrostatic unit is geared to the second row sun of the torque splitting planetary via second clutch and a second pair of gears in Modes Two and Three. A third clutch connects the planet carrier to the output in Modes One and Two. A fourth clutch connects the second planetary row sun gear to the output in Modes Three and Four. Additionally, there is a reverse brake attached to the third planetary row. This planetary row has six planets, three of which mesh with the third row sun, but not the ring, and three of which mesh with the ring but not the sun. Each planet which meshes with the sun gear also meshes with one of the planets which meshes with the ring gear. Accordingly, when the reverse brake is engaged and both the third and fourth clutches are disengaged the third row sun and thus the transmission output is driven in reverse.
The foregoing and other objects of the invention together with the advantages thereof over the known art which will become apparent from the detailed specification which follows are attained by a hydro-mechanical transmission comprising: an input shaft adapted to be connected to a rotational power source; an output shaft adapted to be connected to a load; a hydrostatic transmission including: a first hydrostatic element; a second hydrostatic element in fluid communication with the first hydrostatic element; a mechanical transmission having a planetary gear system including: a first clutch, a second clutch, a third clutch and a fourth clutch; a first element coupled to the input shaft and driven thereby; a second element in driving engagement with the first hydrostatic element; a third element selectively coupled to the second hydrostatic element by the first clutch; a fourth element selectively coupled to the second hydrostatic element by the second clutch, the fourth element drivingly coupled to the second element; a fifth element coupled to the output, the fifth element being selectively coupled to the third element by the third clutch, and selectively coupled to the fourth element by the fourth clutch; and, a sixth element selectively coupled to ground by a reverse brake wherein the fifth element and the output shaft are driven in reverse.
Other objects of the invention are attained by a hydro-mechanical transmission comprising: an input shaft adapted to be connected to a rotational power source; an output shaft adapted to be connected to a load; a hydrostatic transmission having first and second hydrostatic elements in fluid communication with one another; a mechanical transmission having a planetary system with three planetary gear sets, the mechanical transmission having a plurality of input elements with one input element coupled to the input shaft and continuously driven thereby and the other input elements selectively coupled to the second hydrostatic element by two clutches, the mechanical transmission combining power from the plurality of input elements into a single hydro-mechanical power output connected to the output shaft by two clutches; and, a reverse brake selectively coupled to a planetary gear set of the mechanical transmission; whereby the transmission has four forward speed modes and two reverse speed modes for infinitely variable speed adjustment between a full speed reverse and a full speed forward.
Still other objects of the invention are attained by a hydro-mechanical transmission comprising: an input shaft adapted to be connected to a rotational power source; an output shaft adapted to be connected to a load; a planetary gear system having a first planetary gear set, a second planetary gear set, and a third planetary gear set, each planetary gear set comprising a sun gear and a plurality of planet gears, each planet gear of the first planetary gear set being integrally formed with a planet gear of the second planetary gear set so as to rotate together, the planet gears of each planetary gear set being mounted on a common planet carrier, the second and third planetary gear sets each further comprising a ring gear; the input shaft drivingly connected to the sun gear of the first planetary gear set; the planet carrier selectively connected to the output by a third clutch; the sun gear of the second planetary gear set being selectively connected to the output by a fourth clutch; a second hydrostatic element selectively geared to the planet carrier via a first clutch and a gear pair, and selectively geared to the sun gear of the second planetary gear set via a second clutch and a second gear pair; a first hydrostatic element geared to the ring gear of the second planetary gear set, the first hydrostatic element being in fluid communication with the second hydrostatic element, the first and second hydrostatic elements being capable of selective variable displacement such that increasing the displacement of the first hydrostatic unit reduces the displacement and increases the speed of the second hydrostatic unit and vice versa; and, a reverse brake selectively connected to the ring gear of the third planetary gear set; wherein the first and third clutches are engaged, the displacement of the first hydrostatic element is increased to a maximum and the displacement of the second hydrostatic element is decreased to zero for a first output mode, the first clutch is disengaged, the second clutch is engaged, the displacement of the second hydrostatic element is increased in an opposite direction to that of mode one and the displacement of the first hydrostatic element is decreased until all parts of the planetary gear system are rotating at the same speed for a second output mode, the third clutch is disengaged, the fourth clutch is engaged and the displacement of the first hydrostatic element is increased to a maximum and the displacement of the second hydrostatic element is decreased to zero for a third output mode, the second clutch is disengaged, the first clutch is engaged, the displacement of the second hydrostatic element is increased to a maximum in a direction opposite to that of the third mode and the displacement of the first hydrostatic element is decreased to zero for a fourth output mode, and the reverse brake is selectively engaged in the first and second output modes to obtain reverse.
Still further objects of the invention are attained by an agricultural tractor comprising: an engine; drive wheels; a hydro-mechanical transmission driven by the engine and drivingly coupled to the drive wheels, the hydro-mechanical transmission comprising: an input shaft adapted to be connected to the engine; an output shaft adapted to be connected to the drive wheels; a hydrostatic transmission including: a first hydrostatic element, a second hydrostatic element in fluid communication with the first hydrostatic element; a mechanical transmission having a planetary gear system including: a first clutch, a second clutch, a third clutch and a fourth clutch; a first element coupled to the input shaft and driven thereby; a second element in driving engagement with the first hydrostatic element; a third element selectively coupled to the second hydrostatic element by the first clutch; a fourth element selectively coupled to the second hydrostatic element by the second clutch, the fourth element drivingly coupled to the second element; a fifth element coupled to the output, the fifth element being selectively coupled to the third element by the third clutch, and selectively coupled to the fourth element by the first clutch; and, a sixth element selectively coupled to ground by a reverse brake wherein the fifth element and the output shaft are driven in reverse; whereby the transmission has four forward speed modes and two reverse speed modes for infinitely variable speed adjustment between a full speed reverse and a full speed forward.
To acquaint persons skilled in the art most closely related to the present invention, one preferred embodiment of the invention that illustrates the best mode now contemplated for putting the invention into practice is described herein by and with reference to, the annexed drawings that form a part of the specification. The exemplary embodiment is described in detail without attempting to show all of the various forms and modifications in which the invention might be embodied. As such, the embodiment shown and described herein is illustrative, and as will become apparent to those skilled in the art, can be modified in numerous ways within the spirit and scope of the inventionxe2x80x94the invention being measured by the appended claims and not by the details of the specification.